1. Field of the Invention
This invention relates to methods and systems for laser processing a workpiece and methods and apparatus for controlling beam quality therein. An improved laser source has particular applicability to laser welding, cutting, scribing, and similar applications.
2. Background Art
Continuous wave lasers (CW lasers) are commonly used in material processing applications. These are typically solid state lasers and may have Nd:YAG laser rods (laser media) or other types of laser rods. They are commonly used for material processing operations, such as drilling, welding, cutting, ablation, heat treatment and so on, on both metal and non-metal target materials.
Referring to FIG. 1, a CW laser depicted therein comprises a laser resonator 1 including at least one laser rod 2 (typically an Nd:YAG element) which is mounted between two flat mirrors 3 and 4. The mirror 3 is a high reflectivity rear mirror 3 and the mirror 4 is a partially transmissive front mirror 4, known as the output coupler. The laser rod 2 is pumped by one or more pumping elements such as lamps 5, which are powered by an electrical source 6 (e.g. AC source) that generally includes a resonant circuit. Typically the source 6 is designed to produce an output of about 15 kilowatts average power and 30 kilowatts peak power.
CW lasers have a rated average power and this is shown in FIG. 2 as level “CW.” This level may be, for example, 1000 watts, as shown in FIG. 2. Such a CW laser is generally modulated by altering the power supplied to one of more pumping lamps to a level up to the CW level depending upon the power required at any time during a material processing operation. As shown in FIG. 2, the level may be dynamically varied up to the CW level to control a processing operation. Alternatively, a DC power supply may be used, but such use should be carefully considered as failure rates may increase. The CW output is one that can be maintained for 100% of the time as an average level and the laser power is modulated up to this. A parameter known as “process speed” is, in many cases, determined by the output power and beam quality at any time.
It is sometimes desirable to try to achieve as fast a processing time as possible and, up to now, this has been done by using either high levels of modulation or high beam quality. High modulation is generally achieved with very high frequencies of well above 1500 Hz using effects like Q-switching, phase-locking, acousto-optic modulation and so on. These effects, which are done at high modulation frequency, are designed to produce high single peak but lower average power laser outputs. For example, a high power q-switched laser may have high peak power at repetition frequencies of a few KHz, but average power orders of magnitude less. The pulse width may be substantially less than 1 microsecond resulting in energy too low for high power processing. Conversely, at very high repetition rates the average power increases, but with an accompanying decrease in peak power, stability, and pulse energy.
The teaching of present laser technology, particularly high power CW laser technology, is to try to improve beam quality as much as possible. Beam quality is defined by the size of a spot (or a diameter of a waist in a laser beam) and is generally measured in mm-mrad. Beam quality of a laser beam is determined by the full angle divergence of the beam times the diameter of the beam at a waist position (all references herein refer to the diameter and full-angle divergence criteria). The lower the numerical value of the beam quality, the better for most applications, although there is generally a trade-off between beam quality and output power. In various CW systems, processing speeds are based on beam quality improvements. For example, excellent beam quality has been obtained in diode pumped CW systems having limited output power.
It is well known that the worst-case beam quality of a laser is inversely proportional to the effective length of the resonator, and proportional to the area of the limiting aperture inside the resonator, which is usually the rod diameter. The best beam quality (i.e., the lowest value) is therefore derived from long resonators with small radius limiting apertures. Although some resonators have single rods as shown in FIG. 1, often resonators have a plurality of rods arranged in series.
Various methods are known for controlling beam quality of high power solid state lasers having one or more rods. Several methods are described in “LIA Handbook of Material Processing” pp. 42–44, 2001. For example, in lasers with single rods, the beam quality can be varied by moving the output coupler and rear mirror either further apart or closer together to improve or degrade beam quality, respectively. However, in periodic resonators with more than one rod, this is not possible. In such cases, the pumping chambers have to move to maintain periodicity and symmetry. This is usually impractical due to water and power connections.
An alternative method of controlling beam quality is to vary the radius, and therefore area, of the limiting aperture in the resonator. In most industrial lasers, as described, the beam quality-defining aperture is the rod. The beam quality can then be controlled by using different diameter rods. Small diameter rods can be used for high beam quality and larger diameter rods for poorer beam quality.
Typically, improved beam quality is at the expense of lower output power and therefore a balance has to be made. However, a limitation of this technique is that to obtain very good beam quality, a very small diameter rod is required if the pitch of the resonator is to be limited to practical values. To obtain high power, long rods are required. Long thin rods, however, have poor stiffness and can be flexed easily by coolant flow causing instability in the laser output. Such rods, being very long and very thin, are subject to breakage when mechanically stressed in handling.
A further alternative of controlling beam quality is to place apertures midway between the rods in a periodic resonator or to place them close to the output coupler or rear mirror. It is known that waists are formed in periodic resonators, and by placing an aperture at a waist position, beam quality can be controlled. One limitation of this particular technique is that the waist diameter, and therefore aperture transmission, depends on the value of thermal lensing in the laser rods. At low pump power, and therefore low thermal lensing, the diameter of the waist is at its largest and this decreases as pump power increases.
This impacts the rise time characteristics when the laser is switched from cold (or lower power) to high power. The apertures have lower transmission until the rods reach thermal equilibrium thus limiting the rise time of the laser output to about one second, for example. Many welding and cutting processes require a laser rise time much faster than this, typically of 1 to 10 ms. This precludes the use of such ‘far field’ apertures. A further limitation is that such apertures have to be centered with respect to the axis of the laser rods and also have to be cooled. This adds cost and complexity to the laser design.
Hence, an improved laser apparatus for use in high power system would provide beam quality management in a manner that is less sensitive to thermal variations in a particular laser arrangement (e.g. single rod and periodic resonators). Such an improvement could be applied to pulsed and CW systems.
Another current tenent of laser technology presumes that modulating a beam at low levels would only present an advantage with low beam quality equipment (i.e. numerical values significantly greater that 100 mm mrad). It has therefore always been assumed, up to now, that either high beam quality, or modulation is required.
While present laser apparatus work reasonably well, there is always room for improvement and efficiency in terms of cost and better material processing and other uses of laser apparatus. The present invention arose in an attempt to provide an improved laser apparatus for laser processing, so that higher processing speeds may be obtained with improved workpiece processing results.